Project Page

http://www.SignalONE.com/radioastronomy/telescope/

5.2 Meter Radio
Astronomy Project for 1420 MHz

This is a description of my
1420 MHz radio telescope project for observing the
natural radio emissions of neutral hydrogen atoms found
throughout space. Specifically, the study of the radio
spectra of these emissions is used to determine the
distribution and dynamics of hydrogen throughout our
galaxy.

Hello, Everyone!

I have been interested in radio astronomy for
years but have only recently been able to find the time
to assemble my own radio telescope. I have completed
construction of a 5.2 meter antenna which I use to detect
the 1420 Mhz radio emissions of neutral hydrogen found
throughout the galaxy. I have included some information
on construction and sources of material, and there is
also a description of how I am using it and what type of
information can be gained from these types of
observations. It is my hope that this information will be
helpful to others who are interested in radio astronomy
or who may even be considering a project of their own.

The antenna has a
computer controlled Az-El mount and was built mostly from
recycled C-band satellite TV components. The elevation
drive was once a horizon-to-horizon drive for a 16-foot
satellite TV antenna and has an integral worm drive. It
is driven by a 90-vdc linear actuator motor/gearbox. The
azimuth drive is a roller-chain arrangement driven by
another worm drive. This worm drive is also driven by a
linear actuator motor/gearbox. The antenna uses a
potentiometer for azimuth position feedback which
produces a voltage ratio read by a "data
acquisition" board on an old 286 PC. Elevation
position is obtained by using a digital inclinometer connected to one of the 286 PC COM
ports.

Electronics
include a 1420 Mhz GaAsFET preamp, an ICOM R-7000
receiver (AGC disabled), and a Tektronix 2710 digital/storage
spectrum analyzer (10 KHz-1800 MHz). The spectrum
analyzer can be controlled by an old HP-87 microcomputer
which I use as a low cost IEEE-488/GPIB interface. The
HP-87, 286 PC and an Apple Mac (used for data collection
and reduction) all communicate via RS-232 links. More
infomation can be found on the block diagrams shown
below.

What this Project is All About:

OBJECTIVES

This project is a hobby that combines mechanical,
electrical, electronic, radio frequency, astronomy,
physics, math, and software challenges. Some proficiency
is required in all of the areas in order to obtain
meaningful results. It has been fun and VERY educational!

I find it interesting to observe and confirm some of
the original observations made by radio astronomy
professionals in years past. It is gratifying to compare
my results with what I find in the reference books, and
in a small way share the excitement of the early
researchers. I read their books and can duplicate some of
their experiments. Who knows? Maybe I will stumble across
something they missed!

I never get bored and am always looking for ways to
improve the quality of the data, such as through improved
receiver sensitivity, better antenna tracking accuracy,
or better software.

The bad news is that I have no external funding. No
grants or advances. Everything is bought out of the
family budget! The good news is that I have no exernal
funding! This means no deadlines, no publication dates,
no pressure. I can take my time, focus on the areas that
interest me the most, and enjoy!

(click on images for
larger views and more detailed explanations)

A
typical spiral galaxy (M100 in Coma Berenices) similar to
the Milky Way. If this were our own galaxy the sun would
be located near the outer edge of an arm. Hydrogen is the
most abundent element in the galaxy. (NASA/HST photo)

A
simplified diagram of the Milky Way galaxy. The sun is
located about 30,000 light years (approx. 9 kiloparsecs)
from the center. (Ref. 1)

After
being excited, a neutral hydrogen atom will spontaneously
decay, on average, after about 12 million years and emit
a photon on 1420.40575 MHz. Fortunately, hydrogen is
found in great abundance throughout the galaxy and these
emissions are sufficient to be detected and studied. This
is a crude map of neutral hydrogen in our galaxy derived
from radio telescope observations. (Ref. 1)

The
motion of different parts of the galaxy relative to the
sun caused by the spiral rotation. Studies of the galaxy
usually refer to galactic latitude/longitude coordinates.
By convention, galactic longitude (l) is zero towards the
center and 180 degrees directly away from the center.
(Ref. 1)

The
rotation curve for the galaxy showing rotation velocity,
v (km/sec), and rotation rate, w, (radian/sec). This
motion introduces significant doppler shifts in original
emission frequency of 1420.40575 MHz. The frequency
spectrum of these emissions reveals information on the
structure and dynamics of the galaxy, as shown below.
(Ref. 1)

Example
radio spectra of hydrogen emissions (frequency vs.
strength). (a): If a source were stationary emissions
would be received only on a single frequency (vertical
line in center). Practical receiver bandwidths broaden
the spectral line into a typical curve. (b): Effect of a
cloud moving away from the observer. The received
spectrum is shifted down in frequency. (c): Turbulence
within a cloud broadens the received spectrum. (d): A
typical hydrogen spectrum is composed of emissions from
multiple sources moving at different rates. (Ref. 1)

Typical
hydrogen emission spectra from four different galactic
longitudes (l). The received frequency (relative to the
rest frequency) and corresponding relative velocity of
the sources can be determined. (Ref. 1)

Simplified
analysis of a received spectrum (l=75 deg). If the motion
of hydrogen in the galaxy was perfectly circular, and
followed the distance-velocity (solid curve) shown in
(a), the expected idealized received spectrum would be
the solid curve in (b). If the hydrogen were of uniform
density, temperature, and dispersion the expected
spectrum would be the solid curve in (c). The dashed
lines show the effect of variations in local velocities
of a few km/sec. The dotted curve in (c) shows an
observation. (Ref. 2)

Typical Results:

Typical scans in the frequency range of
1420 to 1421 MHz plotted from the spectrum analyzer.
These spectra of hydrogen line emissions show predominant
peaks at different frequencies depending on the galactic
longitude where taken. All of the emissions originated at
the rest frequency of 1420.40575 MHz and have their
spectrum broadened by turbulence.

In
addition, observed spectra are shifted due to motions of
the Earth's rotation, the motion of the Earth around the
Sun, and the motion of the Sun through the galaxy. These
motions must be calculated and taken into account in
order to convert frequency into relative velocity.

A
doppler shift of +100 KHz represents a closing velocity
of the source of about 21.1 Km/sec.

Many
of these scans will be necessary to form a complete
database for the galaxy. The plots shown are actually 16
or 32 individual spectra averaged. This process helps to
cancel random noise in the data.

Click
on the individual images for larger views. The value of
"l" indicates the galactic longitude of the
observation.